The present invention relates to a process and to an apparatus for obtaining a gaseous stream containing a compound in the vapour state, more particularly usable for introducing said compound in to a vapour phase epitaxy reactor.
Vapour phase epitaxy methods are generally used for producing semiconductors and particularly those of the III-V or II-VI types.
This method can in particular be used for forming heterojunctions constituted by several layers of semiconductors having different compositions, which have numerous applications, e.g. in optoelectronics (lasers, photodetectors) and in microelectronics (highspeed transistors).
For the vapour phase epitaxy deposition of composite semiconductor layers, the cationic constituents of the layer are generally introduced into an epitaxy enclosure in the form of vapours of their organometallic compounds.
Thus, in the case of semiconductors of the III-V type, such as gallium arsenide GaAs or indium phosphide InP, the growth rate of the semiconductor compound deposited by vapour phase epitaxy firstly depends on the partial pressure of the organometallic compounds containing the cationic constituents (Ga, In) at the epitaxy cell intake.
In the case of semiconductor compounds such as Ga.sub.1-x In.sub.x As.sub.y P.sub.1-y, the cationic composition x in the solid phase is also directly dependent on the ratio of the partial pressures of the corresponding organometallic compounds in accordance with the following simple relation: ##EQU1## in which pGa and pIn are respectively the partial pressures of the organometallic compounds containing the gallium and the indium, whilst K is a constant which is relatively insensitive to the other parameters of the process, such as the substrate temperature or the total gaseous flowrate.
However, in the process of growing alloys of type Ga.sub.1-x In.sub.x As.sub.y P.sub.1-y, it is vital to control the composition x within very narrow limits. Thus, a variation of approximately 1% is prohibitive, because it leads to a crystal mesh parameter variation .DELTA.a/a of approximately 10.sup.-3, which is generally sufficient to bring about an irreparable deterioration to the electronic properties of the material.
Moreover, in processes of this type, it is very important to be able to accurately control the partial pressures of the organometallic compounds introduced into the epitaxy reactor.
Hitherto, the conventional procedure for introducing these organometallic compounds into the epitaxy reactor has consisted of bubbling an inert carrier gas into the compound in the liquid state, so as to charge the carrier or vector gas in the vicinity of the vapour saturation of said compound. The saturated carrier gas is then diluted, in order to obtain the desired partial pressure.
An apparatus making it possible to obtain in this way a gaseous stream containing an organometallic compound in the vapour state is illustrated in FIG. 1. This apparatus comprises a tight enclosure 1 containing the organometallic compound in the liquid state, which can be raised to the desired temperature by a thermostatically controlled bath 3. A pipe 5 connected by means of a valve 6 to a carrier gas source 7 issues into the bottom of enclosure 1. The carrier gas introduced by pipe 5 can be charged with vapour or the organometallic compound by bubbling into the compound in the liquid state, followed by discharge from the enclosure by a pipe 9 provided with a valve 10. The gaseous stream is then introduced into the epitaxy reactor by pipe 11 after diluting it to the desired value by adding carrier gas supplied by pipe 13, equipped with the valve 14 connected to the carrier gas source 7.
When using an apparatus of this type, the partial pressure of the organometallic compound in the carrier gas leaving by pipe 11 is dependent on the following physical parameters:
(1) the temperature of the source of the organometallic compound in enclosure 1; PA1 (2) the flowrate of the carrier gas bubbling into the organometallic compound source; PA1 (3) the gaseous phase pressure above the organometallic compound source in the enclosure 1; PA1 (4) the liquid level in enclosure 1; PA1 (5) the total flowrate and the pressure of the gas in the epitaxy reactor.
Among these different parameters, only the total flowrate and pressure of the gas in the epitaxy reactor acts simultaneously and in the same way on all partial pressures of the organometallic compounds introduced into said reactor. Thus, they are not likely to modify the solid phase composition, which is solely a function of the ratios between the different partial pressures. However, the first four parameters act independently on each partial pressure and for accurately checking the composition of the solid phase alloys, it is necessary to accurately control each temperature, each carrier gas flowrate and each gas pressure in contact with the sources of organometallic compounds.
In order to check the gaseous flowrates, use is generally made of a mass regulating flowmeter, which is a flowrate transducer based on the heat transfer measurement in the gas, coupled via an electronic regulating circuit to a variable opening valve.
Although such equipment represents a considerable advance compared with older procedures, such as ball flowmeters, they suffer from the following disadvantages. They are sensitive to the temperature of the gases and to ambient temperature, typically 0.1% of the full scale per .degree.C. Therefore variations of only a few degrees can compromise the quality of an epitaxy operation by modifying the composition of an alloy by a fraction of a percent. They have a high cost and only mediocre reliability due to their complexity. They have relatively long response times (a few seconds), so that it is difficult to ensure that the epitaxy process is controlled in transient operating conditions, e.g. the interface between two layers of different compositions.
The use of an apparatus of this type also suffers from other disadvantages. Thus, the carrier gas on leaving enclosure 1 is almost charged with organometallic compound, so that during its passage through a duct at a temperature below that of the organometallic compound source, organometallic compound can be deposited. To avoid and minimise this highly disadvantageous effect, the ducts are generally heated and, as soon as possible, the carrier gas is diluted, which makes the equipment much more complicated and inter alia leads to a doubling in the number of flowmeters.
Furthermore, this apparatus cannot be used for all organometallic compounds, because it is necessary for the combination of the physical properties of the organometallic compound, in particular its melting point and vapour pressure to be suitable for putting into effect this procedure for evaporating and introducing into a gaseous stream.
Thus, certain compounds can have an excessively high vapour pressure or tension at temperatures close to ambient temperature, which involves cooling them to lower temperatures. Other compounds can have a suitable vapour pressure, but can be in the solid state at temperatures close to ambient temperature.
Although it is possible to saturate a carrier gas by circulating it above a solid which sublimates, said process in practice leads to less reproduceable results than those supplied by bubbling gas into a liquid source. In addition, it is necessary to very carefully select the compounds to be introduced by this procedure into the epitaxy reactor.